selinux: call WARN_ONCE() instead of calling audit_log_start()

Two of the conditions in selinux_audit_rule_match() should never happen and
the third indicates a race that should be retried.  Remove the calls to
audit_log() (which call audit_log_start()) and deal with the errors in the
caller, logging only once if the condition is met.  Calling audit_log_start()
in this location makes buffer allocation and locking more complicated in the
calling tree (audit_filter_user()).

Signed-off-by: Richard Guy Briggs <rgb@redhat.com>
Signed-off-by: Eric Paris <eparis@redhat.com>